Investigation of epidemiological and molecular factors associated with synovial infections in horses and foals

Dani Crosby

Research output: ThesisDoctoral Thesis

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Abstract

Synovial infections (SI) are common in horses of all ages and can be associated with high rates of morbidity and mortality. Early recognition of the condition and prompt institution of appropriate therapy is considered vital for positive outcomes. Unfortunately, definitive diagnosis of SI in horses can be problematic. The preferred test for confirmation of bacterial involvement in synovial disease is microbiological culture; however, this method can have low sensitivity. The limitations associated with conventional bacterial culture may prevent evidence-based antimicrobial selection. An understanding of factors associated with survival and return to function of horses with SI is also important for evidence-based treatment decisions, and for accurate prognostication of affected horses. While there are a number of studies that have examined factors associated with prognosis for horses with SI, there is a lack of agreement in risk factors for outcomes of SI between studies. Furthermore, there are limited studies from the geographical region of south-eastern Australia, and regional differences may preclude translation of findings from studies undertaken in other horse populations.
The aims of the research presented in this thesis were to: (i) investigate epidemiological factors associated with septic synovial structures in horses in south-eastern Australia and (ii) investigate molecular techniques for the diagnosis of SI in horses. Epidemiological factors associated with septic synovial structures (part i) were investigated by (i.i) describing outcomes for horses and foals with SI presented to the Veterinary Clinical Centre (VCC), Charles Sturt University (CSU) and (i.ii) identifying factors associated with survival to discharge from hospital and return to function for horses and foals with SI presented to the VCC, CSU. Molecular techniques for the diagnosis of SI in horses (ii) were investigated by (ii.i) determination of an appropriate protocol to optimise bacterial DNA extraction from equine septic synovial fluid (SF), (ii.ii) determination of the efficacy of different 16S rRNA primers for application to equine septic SF and (ii.iii) comparison of the effectiveness of a broad range PCR method to microbiological culture for diagnosis of SI in horses.
For the epidemiological study (part i), a retrospective case-control study was used to investigate the outcomes of horses presented to the VCC with SI. One-hundred and eight six horses were presented during the study period with SI and 161 were treated. The overall rate of survival for horses treated was 90.1% (145/161) and return to function was lower than survival at 65% (79/121). Increasing number of days of treatment with systemic antimicrobial drugs increased the likelihood of survival for affected horses. Treatment with doxycycline decreased the likelihood of horses returning to function. However this finding may be explained by the use of this drug in juvenile horses and/or those with infections refractory to treatment with alternative antimicrobial drugs preferentially receiving doxycycline. The findings of this study, can be used as a part of evidence-based decision-making when veterinarians are treating horses with SI.
The results of the molecular studies (part ii), demonstrated that DNA extraction of clinical equine SF samples being investigated for sepsis was possible using the commercial kit investigated. However, the results of these studies highlighted that inflammatory debris within septic SF samples may impede extraction protocols and some modifications to protocols in commercially provided kits may be required to allow successful DNA extraction. The equine β-actin gene was determined to be an effective positive control for broad range PCR methods applied to equine SF. Broad range 16S rRNA PCR was determined to be able to detect microorganisms in septic SF samples from horses. The sensitivity of this technique was modest but comparable to the use of microbiological culture in the population. Synovial fluid 16S rRNA PCR does not replace the use of microbiological culture; however, the method may be a useful adjunct in a clinical setting.
This thesis provides new information on the understanding of outcomes associated with treatment of SI in horses, this information can assist veterinarians with evidence-based decision making for horses with the condition in an Australian context. Additionally, this study has provided contributions to the knowledge of the use of molecular techniques for the diagnosis of SI in horses, including the variable sensitivity of the techniques used. Further, the results of this study support that molecular methods may be of greatest benefit when used concurrently with microbiological culture, rather than a sole diagnostic method. The use of likelihood ratios during the interpretation of results of both 16S rRNA PCR and conventional bacteriological culture were also described.
Original languageEnglish
QualificationDoctor of Veterinary Studies
Awarding Institution
  • Charles Sturt University
Supervisors/Advisors
  • Hughes, Kris, Principal Supervisor
  • Labens, Raphael, Co-Supervisor
  • Hilbert, Bryan, Principal Supervisor
Place of PublicationAustralia
Publisher
Publication statusPublished - 2020

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